Though news organizations and fact-checking Web sites like Snopes.com have debunked the claim, the story just wonít die. In an NBC/Wall Street Journal poll taken in December, 8 percent of respondents thought Obama was Muslim, half as many as correctly identified him as a Protestant.

The Obama-is-a-Muslim rumor does not seem to have hurt the candidateís fortunes, at least not yet. But the mythís persistence illustrates a growing cultural vulnerability to rumor. Journalists typically presume that facts matter: show the public what is true, and they will make decisions correctly. Psychologists who study how we separate truth from fiction, however, have demonstrated that the process is not so simple. And because digital technology fosters social networks that are both closely knit and far-flung, rumors are now free to travel widely within certain groups before they meet any opposition from the truth.